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Page:Popular Science Monthly Volume 4.djvu/632

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comes transformed into a granular, opaque, bulbous ice, called névé ice, which at last changes into the compact blue ice of the glacier proper.

During the summer, when the snow-fall is at its minimum, the surface of the snow in the high regions becomes covered with dirt and sand, which the next winter covers with another snow-sheet. In summer these layers of snow, from a partial melting and subsequent freezing, become changed to ice on their surfaces, so that we have three kinds of deposits—beds of snow, sheets of dust, and layers of ice. As the whole is pushed down into the valley, these layers tend to assume a vertical position from the bottom of the mass, moving faster than the top; and, the snow in summer melting from the surface as far as the snow-line, the edges of the layers are found passing transversely across the glacier. The middle of the glacier being deeper, moves faster than the sides, and, the lower layers advancing more rapidly than the upper ones, the strata become curved forward, the lower layers being more sharply arched. The arch thus becomes the measure of the rate of movement in the different parts of the glacier. From this it will be seen that Agassiz dissents from the theory of Tyndall, which represents the stratified lines as due to ice-cascades, or breakages of the glacier in passing over sharp angles.

All glaciers exhibit numerous blue bands, which are parallel to the planes of stratification, and are formed by thawing and freezing, and by the vertical pressure of snow in the névé. Moreover, there are found certain veined structures of the ice which appear to be bands of infiltration, and intersect the planes of stratification at every possible angle. As they are most numerous at the sides of the glacier, it is probable that Tyndall's theory of internal liquefaction of ice by pressure may account for them.

In the progress of the glacier, its rate of movement is not uniform, the differences between the centre and the sides being about as ten to one. Neither is the motion uniform along the axis; the advance being greatest about half-way down the region of the névé, and diminishing in rapidity both above and below. Agassiz found that it was from 20 to 50 feet per year in the higher portions, about 250 feet in the névé, and diminishing again lower down.

The causes of the movement of the glacier are several. The weight of the glacier alone is not sufficient to propel it, as in this case the greatest movement would be in the winter, which is not the case. The principal agent is the infiltration of water, which is greatest when the winter snows are melting. The granular snow of the névé incloses numerous particles of air, which, when the snow is compressed into ice, form a net-work of capillary fissures that serve as canals of infiltration, the water in which, freezing, aids in propelling the glacier. Added to this may be such other causes as the weight of the mass, the pressure of accumulated snow above, the weight of infiltrated water, and the softening of the ice by water, and a consequent sliding along the surface.